Huang Yang, Sun Shaolong, Huang Chen, Yong Qiang, Elder Thomas, Tu Maobing
College of Chemical Engineering, Nanjing Forestry University, Nanjing, 210037 China.
Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, 2901 Woodside Drive, Cincinnati, OH 45221 USA.
Biotechnol Biofuels. 2017 Jun 24;10:162. doi: 10.1186/s13068-017-0853-6. eCollection 2017.
Lignin typically inhibits enzymatic hydrolysis of cellulosic biomass, but certain organosolv lignins or lignosulfonates enhance enzymatic hydrolysis. The hydrophobic and electrostatic interactions between lignin and cellulases play critical roles in the enzymatic hydrolysis process. However, how to incorporate these two interactions into the consideration of lignin effects has not been investigated.
We examined the physicochemical properties and the structures of ethanol organosolv lignins (EOL) from hardwood and softwood and ascertained the association between lignin properties and their inhibitory and stimulatory effects on enzymatic hydrolysis. The zeta potential and hydrophobicity of EOL lignin samples, isolated from organosolv pretreatment of cottonwood (CW), black willow (BW), aspen (AS), eucalyptus (EH), and loblolly pine (LP), were determined and correlated with their effects on enzymatic hydrolysis of Avicel. EOLs from CW, BW, and AS improved the 72 h hydrolysis yield by 8-12%, while EOLs from EH and LP decreased the 72 h hydrolysis yield by 6 and 16%, respectively. The results showed a strong correlation between the 72 h hydrolysis yield with hydrophobicity and zeta potential. The correlation indicated that the hydrophobicity of EOL had a negative effect and the negative zeta potential of EOL had a positive effect. HSQC NMR spectra showed that β--4 linkages in lignin react with ethanol to form an -ethoxylated β--4' substructure (A') during organosolv pretreatment. Considerable amounts of C-H correlation in -hydroxybenzoate (PB) units were observed for EOL-CW, EOL-BW, and EOL-AS, but not for EOL-EH and EOL-LP.
This study revealed that the effect of lignin on enzymatic hydrolysis is a function of both hydrophobic interactions and electrostatic repulsions. The lignin inhibition is controlled by lignin hydrophobicity and the lignin stimulation is governed by the negative zeta potential. The net effect of lignin depends on the combined influence of hydrophobicity and zeta potential. This study has potential implications in biomass pretreatment for the reduction of lignin inhibition by increasing lignin negative zeta potential and decreasing hydrophobicity.
木质素通常会抑制纤维素生物质的酶水解，但某些有机溶剂木质素或木质素磺酸盐可增强酶水解。木质素与纤维素酶之间的疏水和静电相互作用在酶水解过程中起关键作用。然而，如何将这两种相互作用纳入对木质素作用的考量尚未得到研究。
我们研究了来自阔叶木和针叶木的乙醇有机溶剂木质素（EOL）的物理化学性质和结构，并确定了木质素性质与其对酶水解的抑制和促进作用之间的关联。测定了从杨木（CW）、黑柳（BW）、白杨（AS）、桉木（EH）和火炬松（LP）的有机溶剂预处理中分离得到的EOL木质素样品的ζ电位和疏水性，并将其与它们对微晶纤维素酶水解的影响相关联。来自CW、BW和AS的EOL使72小时水解产率提高了8 - 12%，而来自EH和LP的EOL分别使72小时水解产率降低了6%和16%。结果表明72小时水解产率与疏水性和ζ电位之间存在很强的相关性。该相关性表明EOL的疏水性具有负面影响，而EOL的负ζ电位具有正面影响。HSQC NMR光谱表明，在有机溶剂预处理过程中，木质素中的β - O - 4键与乙醇反应形成了一个乙氧基化的β - O - 4'亚结构（A'）。在EOL - CW、EOL - BW和EOL - AS中观察到了大量在对羟基苯甲酸酯（PB）单元中的C - H相关性，但在EOL - EH和EOL - LP中未观察到。
本研究表明，木质素对酶水解的影响是疏水相互作用和静电排斥共同作用的结果。木质素的抑制作用受木质素疏水性控制，而木质素的促进作用受负ζ电位控制。木质素的净效应取决于疏水性和ζ电位的综合影响。本研究对于通过增加木质素负ζ电位和降低疏水性来减少木质素抑制的生物质预处理具有潜在意义。
